1. Field of the Invention
The present invention relates to a CDMA (code-division multiple-access) multiuser receiver which combines directivity control of an array antenna and interference canceling operations. The CDMA multiuser receiver according to the present invention features a small size and excellent interference cancellation. By way of example, the present invention may be applicable to the receiver installed in the base station of a cellular mobile communications system.
2. Description of the Related Art
It is expected that CDMA is able to markedly increase a subscriber's capacity and thus find an extensive application in a cellular mobile communications system (for example). However, the mobile communications utilizing CDMA (viz., spread-spectrum) techniques have suffered, at a receiver side, the problems of interference caused by delayed signals due to multiple transmission paths and concurrently communicating other party's signals.
As is known in the art, an array antenna is able to suppress and cancel interference through directivity control. On the other hand, a multiuser receiver is a receiver which demodulates all the user's signals by implementing mutual interference cancellation using all the user's spreading codes and channel characteristics. The multiuser receiver itself is known in the art. One example of such a receiver is disclosed in a paper by M. K. Varanasi and B. Aazhang, entitled "Multistage Detection in Asynchronous Code-Division Multiple-Access Communications", IEEE Transactions on Communications, Vol. 38, No. 4, April 1990, pp. 509-519 (Prior Paper 1). Another example of a conventional multiuser receiver is disclosed in a paper by M. Sawahashi, et al., entitled "Serial Canceler Using Recursive Channel Estimation by Pilot Symbols for DS-CDMA", Electronics Information Communications Association of Japan, Technical Report RCS95-50,July 1995 (Prior Paper 2).
According to the apparatus disclosed in the aforesaid Prior Paper 1, all the user's signals are demodulated at an initial stage of the apparatus, after which an interfering replica of each user becomes produced. Subsequently, interference cancellation is implemented by reducing an interference replica of each of the users other than a desired user from a received signal. At the next stage, the signal, which has been obtained through the interference cancellation, is again demodulated in connection with the desired (intended) user and therefore, the signal quality of the demodulation result at the second stage is higher than that at the first stage. Thus, the conventional technique, disclosed in Prior Paper 1, is to improve the interference cancellation by repeating a series of signal processes using multi-stage configuration.
Channel estimation is necessary to demodulate the signal of each user and produce an interference replica. The aforesaid Prior Paper 2 discloses that a channel (viz., transmission path) is recursively estimated at each stage thereby to prevent deterioration of the interference cancellation characteristics due to channel estimation error.
Another example of the multiuser receiver is disclosed in a paper by Yoshida and Ushirokawa, entitled "CDMA Multi-Stage Interference Canceler with Recursive Channel Estimation Based on Symbol Replica Processing", the Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, A. p96-157, EMCJ96-92, RCS96-171, February 1997 (Prior Paper 3).
The above-mentioned Prior Paper 3 discloses a multi-stage type CDMA multiuser receiver. According to this known technique, the size of the apparatus can be reduced through the use of symbol replica processing. At the same time, it is possible to realize interference cancellation at the unit of multi-path in the case of implementing recursive channel estimation thereby to improve interference cancellation in the case of multi-path transmission.
FIG. 1 is a drawing showing a CDMA multiuser receiver that is based on the known techniques disclosed in Prior Paper 3. The CMDA multiuser receiver of FIG. 1 is comprised of three-stage interference cancelers 10-1 to 10-3. At the first two stages of interference cancelers 10-1 and 10-2, the signals of all the users, the number of which is assumed three, are demodulated and then subjected to interference cancellation. That is, this means that the multiuser interference cancellation is implemented.
As shown in FIG. 1, the interference canceler 10-1 at the first stage is provided with a delay unit 12, three EIUs (interference estimation units) 14a-14c, an adder 16, and another adders 18a-18c. The interference canceler 10-2 is configured in the same manner as the canceler 10-1 and is comprised of three IEUs (interference estimation units) 14a'-14c', an adder 16', and another adders 18a'-18c'.
On the other hand, the interference canceler 10-3 at the final stage is provided with IEUs 20a-20c each of which differs from those provided at the first and second stages.
A received signal is directly applied to the first stage (viz., interference canceler 10-1). The interference canceler 10-3 at the final stage is not provided with any delay unit and any adder. The IEUs 20a-20c generate demodulated signals respectively corresponding to the first to third users.
The operations of the interference cancelers 10-1 and 10-2, which are respectively provided at the first and second stages, are identical with each other and thus, there will be described the operation of the first stage. The three IEUs 14a-14c respectively output estimated interference spread signals that are applied to the adder 16. The delay unit 12 operates such as to delay the incoming signal by the time for which each of the IEUs 14a-14c estimates the interference and outputs the result thereof, and applies the output thereof to the adder 16 and the delay unit 12' of the second stage. The adder 16 subtracts the outputs of the IEUs 14a-14cfrom the output of the delay unit 12, and applies the output thereof to the adders 18a-18c that are respectively assigned to the users. Each of the adders 18a-18c sums the output of the adder 16 and the output of the corresponding IEU (14a, 14b, or 14c), and applies the resultant sum to the second stage.
The IEUs 14a-14c of the first stage and the IEUs 14'a-14c' of the second stage are substantially identical with each other in terms of configuration as well as operations, and accordingly there will be described only the IEU 14a of the first stage.
The IEU 14a of FIG. 2 is configured under the assumption that the number of paths of the incoming signal is three (3). In the drawing, the circuits prepared for first to third propagation paths are depicted by P1-P3. Since the circuits for the multiple paths are identical with each other, the description is made with reference to the circuit P1 for the first path. The IEU shown in the drawing is generally comprised of a front section (stage) S1, an intermediate section S2, and a rear section S3. More specifically, the front section S1 comprises a spread-spectrum despreader 22 and a detector 24, while the intermediate section S2 comprises an adder 25 and a discriminator 26. Finally, the rear section S3 comprises a multiplier 27, a spread-spectrum modulator 28 and an adder 29. Further, the detector 24 comprises a channel estimator 24a, a complex conjugate generator 24b, and a multiplier 24c.
The received signal (incoming signal) is split and applied to the circuits P1-P3 prepared for the three transmission paths. The despreader 22 despreads the incoming signal using the first user's spreading code at the timing in synchronism with the spreading code transmitted via the first path, and outputs the operation result.
The detector 24 is supplied with the output of the despreader 22, implementing channel estimation at the channel estimator 24a, applying the estimated channel characteristics to the multiplier 24c via the complex conjugate generator 24b, and implementing carrier phase coherent detection. The multiplier 24c implements amplitude weighting on the output of the despreader 22, using the output of the complex conjugate generator 24b, for the purpose of Rake combination at the subsequent block. The amplitude weighting is for implementing Rake combination (maximum ratio combination) on the output of the despreader 22.
It is deemed advantageous to operate the detector 24, in an environment of fading, using coherent detecting techniques which are disclosed in Prior Paper 2 and via which a carrier is estimated through the use of pilot symbols inserted on a time axis.
The adder 25 combines using Rake combination techniques, the weighted outputs of the multipliers 24c respectively provided in the circuits P1-P3 for the three paths. The combined signal is fed to the discriminator 26 that determines the most likely transmitted symbol.
The output of the discriminator 26 is again split and applied to circuits P1-P3 of the third section S3, which are respectively assigned to the thee transmission paths. The multiplier 27 multiplies the output of the discriminator 26 by the estimated channel characteristics, viz., the output of the channel estimator 24a. The spread-spectrum modulator 28 spreads the output of the multiplier 27 using the first user's spreading code at the timing which is in synchronism with the spreading code transmitted via the first path.
An adder 29 sums (synthesizes) the outputs of the circuits P1-P3 which are respectively assigned to the three paths and which are the replicas of respective paths. Thus, an interference replica of the first user is generated.
The interference canceler 10-3 shown in FIG. 1 is comprised of IEUs 20a-20c which are configured in a manner identical with each other, and accordingly, only the IEU 20a provided for the first user will be described.
Referring to FIG. 3, there is shown the IEU 20a in block diagram form. As shown in FIG. 3, the IEU 20a is configured in a manner exactly identical with those of the front and intermediate sections shown in FIG. 2. Therefore, the reference numerals already used for the blocks of FIG. 2 are attached to the counterparts of FIG. 3 and the description thereof will be omitted.
On the other hand, the techniques for canceling signal interference by applying an array antenna to a CDMA's single user receiver is disclosed in a paper by R. Kohno, H. Imai, M. Hatori and S. Pasupathy, entitled "Combination of an Adaptive Array Antenna and a Canceler of Interference for Direct-Sequence Spread-Spectrum Multiple-Access System", IEEE Journal on selected areas in communications, Vol. 8, N. 4, May 1990, pp. 675-682 (Prior Paper 4).
According to the apparatus disclosed in the aforesaid Prior Paper 4, the array antennas is controlled such as to be directed to an arrival angle of a desired signal and acquires the same, after which the interfering signal components within the directivity are despread. The apparatus demodulates the signal components and generates a temporal symbol, after which the apparatus again spreads the signal and generates interfering signal components. In other words, the apparatus carries out interference cancellation by subtracting the interfering signal components from the signal received by the array antenna, and then demodulates the desired (intended) user's signal. Although this conventional apparatus utilizes spreading codes and channel characteristics of all users, it is understood that the apparatus implements interference cancellation for a single user and thus is classified as a single user receiver.
FIG. 4 shows one example of the above-mentioned conventional receiver wherein an array antenna 30 is combined with an interference canceler. In order to simplify the description and the drawing, it is assumed that an array antenna consists of two antenna elements and the number of total users is three. The receiver is a single user CDMA receiver for demodulating one user (the third user in this particular case).
Superimposed data of desired and interfering signals are applied to two antenna elements 30a and 30b. The signal received at the antenna elements 30a and 30b are respectively weighted, at corresponding complex multipliers 32a and 32b, by antenna weighting coefficients W1 and W2 and thereafter added at an adder 34. The output of the adder 34 is applied to despreader 36a and 36b which are provided for the two users (viz., first and second users) other than the third user (whose signal is to be received in the instant case). Further, the output of the adder 34 is also applied to a delay unit 38. The outputs of the despreader 36a and 36b are respectively applied to discriminators 40a and 40b at which temporal symbol discrimination is implemented. The outputs of the discriminators 40a and 40b (viz., signals representative of temporal symbols) are respectively applied to spreader 42a and 42b which issues interfering signal components based on the discrimination results.
A delay unit 38 is used to delay the output of the adder 34, which is denoted by 34a and is to be applied to an adder 44. In more specific terms, the delay unit 38 is provided to delay the signal 34a (viz, the output of the adder 34) until a signal 34b, applied to the despreader 36a and 36b, is outputted from spreaders 42a and 42b.
The adder 44 subtracts the outputs of the spreaders 42a and 42b (viz., interfering signal components) from the output of the delay unit 38, and applies the result to a despreader 46 and a delay unit 48.
The output of the despreader 46 is applied to a discriminator 50 which demodulates the signal of the third user and outputs the demodulated signal to an external circuit (not shown). That is, the despreader 46 and the discriminator 50 are provided for the third user. On the other hand, the output of the discriminator 50 is also applied to a spreader 52 for the third user, via which a spread signal for the third user is obtained.
The delay unit 48 is to delay the output of the adder 44 (depicted by 44b) by a time period for which the output of the adder 44 (depicted by 44a) has been subjected to symbol discrimination and the spreader 52 generates the spread signal for the third user. The signal thus delayed is applied to the adder 54.
The adder 54 produces an error signal 56 by subtracting the output of the spreader 52 from the output of the delay unit 48. The error signal 56 is fed to an antenna weighting coefficient determiner (adaptively renewing means) 58. This determiner 58 controls the directivity of the array antenna 30 using the signals received at the antenna elements 30a and 30b along with known adaptive algorithm.
The receiver shown in FIG. 4 is an apparatus for use in producing the demodulated signal only for the third user. In other words, in order to demodulate the signals of the other users, viz., first and second users, it is necessary to provide the receivers respectively dedicated to the first and second users.
There has been so far no proposal of combining an array antenna and a CDMA multiuser receiver. By way of example, if an array antenna is simply applied to the multiuser receivers shown in FIGS. 1-3, particularly the interference estimating section becomes complex thereby to be unable to simplify the overall configuration of the receiver.
In addition, the single user receiver shown in FIG. 4, which features a combination of an array antenna and an interference canceler, suffers from the following problem when applied to the case of simultaneously processing a plurality of users. That is, in such a case, it is absolutely necessary to prepare a plurality of identical receivers that are arranged in parallel for respective users.